High-brightness, high-resolution cathode-ray tubes are employed in a variety of applications including, for example, field sequential liquid crystal color displays of the type described in U.S. Pat. No. 4,582,396 to Bos et al. Such displays typically employ a cathode-ray tube having a single electron gun that generates in sequence three color component images that are transmitted through a liquid crystal light shutter to form a full-color display.
These displays are desireable because they are capable of providing high-resolution images in accordance with the resolution of the cathode-ray tube. However, the cathode-ray tube must be capable of forming relatively bright images because the liquid crystal light shutters through which the images are transmitted typically attenuate the brightness of the images by between about 93% and 95%. The cathode-ray tube must, therefore, generate the color component images with a correspondingly increased brightness to provide an acceptable full-color image.
A characteristic of cathode-ray tubes, however, is that increased brightness is provided by an increased electron beam current that typically degrades the display resolution. Specifically, increased electron beam current increases the beam diameter and the space-charge effects among the electrons in the beam, thereby causing the beam to be dispersed and the resolution to be decreased.
U.S. Pat. No. 4,620,134 to Peels et al. discloses a cathode-ray tube having a prefocusing lens and a bipotential main focusing lens positioned in a neck portion of the tube along a central longitudinal axis. The prefocusing lens includes a pair of lens electrodes that are spaced apart such that a factor "S-eff" is less than 1 millimeter, the factor "S-eff" being defined as the minimum of the function: EQU -.DELTA.V/E(z),
in which .DELTA.V is the difference between the operating voltages of the two prefocusing lens electrodes, and E(z) is the electric field strength between the two lens electrodes along the central longitudinal axis of the tube.
The Peels et al. tube purportedly provides improved spherical aberration characteristics. However, the Peels et al. tube exhibits unacceptably high spherical aberration in applications such as display devices that render very bright, high resolution images of greater than about 1,000 lines on display screens with diagonal dimensions smaller than about 33 cm (13 inches). The Peels et al. tube used in such a small display device exhibits relatively high spherical aberration because of the comparatively large beam currents and the relatively small size of the electron-optical elements in the neck portion of the tube.
Moreover, moderately high voltages of about 7 kilovolts employed in the Peels et al. tube can cause arcing within the tube. Arcing typically draws high currents through the cathode-ray tube and associated circuitry, thereby damaging or destroying the circuitry. Tubes with neck portions of relatively small diameter are especially susceptible to such arcing because of the relatively close spacing of the elements in the tubes.